A corrugator that fabricates corrugated fiberboards fabricates a single-face corrugated fiberboard by gluing a corrugated medium and a linerboard together and completes a double-faced corrugated fiberboard by further gluing the single-faced corrugated fiberboard and a top linerboard together. In gluing in a double facer, the single-faced corrugated fiberboard and the top linerboard are previously heated by preheaters immediately before the gluing using a glue.
For example, FIG. 8 is a side view of a typical double facer. As illustrated in FIG. 8, a single-face corrugated fiberboard 3 fabricated through gluing a linerboard (bottom linerboard) 1 and a corrugated medium 2 together by a non-illustrated single facer disposed upstream is preheated by a preheater 11 and is transferred to a double facer 10 after a gluing device 12 applies raw starch solution to the peaks of the corrugated medium 2. In the meantime, a top linerboard 4 is drawn out of a rolled fiberboard 4A mounted on a mill roll stand 20 and is transferred to the double facer 10 after being preheated by a preheater 13.
The double facer 10 includes a heat plate group 14 consisting of a number of heat plate units 14A, having respective horizontal heating surface, arranged in series along the directions, and allows a single-face corrugated fiberboard 3 and a top linerboard being overlaid with the single-face corrugated fiberboard 3, to travel thereon. As shown in FIG. 9, the heat plate group 14 includes a vapor chamber which is supplied with heating vapor by proper means and includes a top surface 21a serving as a dissipating surface for the single-face corrugated fiberboard 3 and the top linerboard 4 (hereinafter collectively called the fiberboard sheet 5A), so that the fiberboard sheet 5A is heated by receiving heat from the top surface 21a. 
As illustrating FIG. 8, over the heat plate group 14, there are disposed an upper belt conveyer 16 and a lower belt conveyer 17, which are extending to downstream of the heat plate group 14. On the backside of the upper belt conveyer 16 at the portion over the heat plate group 14, a pressure device 15 is disposed which presses the single face corrugated fiberboard 3 and the top linerboard 4 by means of, for example, air pressure device or rolls from the top. The wording “flatness” means the magnitude of an gap of a geometrical plane and a surface of a machine part that must be flat, and takes a value representing a minimum distance of two planes both of which are parallel with the representative plane and in which all the points on a measuring surface assigned are existing.
Downstream of the heat plate group 14 and the pressure device 15, a lower roller group 18 that supports the backside of the lower belt conveyer 17 and an upper roller group 19 that is disposed on the backside of the upper belt conveyer 16 are disposed, so that the fiberboard sheet is transferred being interposed between the upper and lower belt conveyers 16 and 17 and being pressed by the upper roller group 19.
The fiberboard sheet introduced between the heat plate group 14 and the pressure device 15 of the double facer 10 travels on the heat plate group 14, being pressed by the upper roller group 19 from the top and thereby, heated by the heat plate group 14. Being heated by the heat plate group 14, the raw starch solution applied to the peaks of the corrugated medium 2 of the single-face corrugated fiberboard 3 is gelatinized so that the adhesion caused from the gelatinization glues fiberboard sheet 5A to thereby fabricate a double-faced corrugated fiberboard 5. The fiberboard sheet 5A travels fast as high as, for example, 300 m/minute and passes through the traveling face of the double facer only for a few seconds.
The double-faced corrugated fiberboard 5 fabricated through the above manner is sandwiched by the upper belt conveyer 16 and the lower belt conveyer 17 from the top and the bottom and then transferred to the subsequent process.
Here, the heating vapor to be supplied to the vapor chamber 21 of the heat plate group 14 normally has a saturated vapor pressure of 1.0-1.3 MPa and a temperature of 180-190° C. The amount of heat and the amount of pressure to be applied to fiberboard sheet 5A on the heat plate group 14 control the adhesion of the fiberboard sheet 5A. Shortage in the amount of heat or pressure to be applied lowers the adhesion and conversely, excess in the amount of heat or pressure to be applied lowers the quality of the double-faced corrugated fiberboard 5 due to flutes formed low.
The heat plate group 14 has to have a width corresponding to the maximum width of a fiberboard traveling thereon and the width normally has a width of 1900-2600 mm. Furthermore, the heat plate group 14 has to uniformly apply heat to the fiberboard sheet 5A and therefore has flatness of 0.1 mm or less, which means high accuracy. In addition, the vapor chamber 21 needs a strength to endure the pressure (1.0-1.3 MPa) of the vapor to be supplied inside thereof, and therefore, each heat plate unit 14A needs to have a bulkhead (rigidity) having a thickness of about 30 mm.
Thickening the bulkhead of the heat plate unit 14A lowers heat conductive efficiency from the vapor in the vapor chamber 21 to the fiberboard sheet 5A. If the temperature of the bulkhead of the heat plate comes to be outside the predetermined temperature range, an amount of heat lacks or exceeds. However, it has been difficult to inhibit such temperature deviation. For the above, a conventional heat plate unit 14A has a bulkhead made of cast iron and having a thickness of about 150 mm with the intention that the bulkhead has a large heat capacity so that the temperature of the bulkhead less varies.
This solution has a problem of low responsibility to the requirement of sharply rising and lowering the temperature caused by variation in a rate of adhering of the fiberboard sheet 5A or variation in kind of paper sheet constituting the fiberboard sheet 5A. Consequently, the adhered portion of the single-face corrugated fiberboard 3 and the top linerboard 4 comes into a state of excessively dried due to excess in heat amount or of incompletely dried due to shortage in heat amount, which causes inferior adhering due to apparent adhering or causes warp of the fabricated corrugated fiberboard. Furthermore, such low responsibility hinders the fiberboard sheet 5A from traveling faster and the productivity cannot be problematically improved.
Adjustment of the temperature of heating the fiberboard sheet 5A is also accomplished by varying a pressure that the pressure device 15 applies to the fiberboard sheet 5A so that the contacting heat transferring efficiency between the fiberboard sheet 5A and the top surface of the heat plate is adjusted. However, such adjustment of the heating temperature that depends on the pressure requires the pressure to vary in a wide range of from a lower state to a higher state. In applying a high pressure to the fiberboard sheet 5A, elements of the pressure device 15 deforms in the width direction of the sheet, which makes it difficult to apply uniform pressure in the sheet width direction to the fiberboard sheet 5A. This unevenness of the pressure causes unevenness of the temperature in the sheet width direction to warp the fiberboard sheet 5A, lowering the quality of the resultant double-faced corrugated fiberboard 5.
In contrast, when the heat plate unit 14A is thinned in such a range that the heat plate unit 14A can endure the pressure of the inside of the vapor chamber 21, no problem related to the strength thereof is caused, but the temperature of the top surface of the heat plate unit 14 lowers as much as an amount of heat that has heated the fiberboard sheet 5A so that the difference in temperature between the top side of the heat plate whose temperature lowers and the bottom side whose temperature does not lower causes the heat plate unit 14A to warp to form a downward convex toward the bottom side heat which is not removed by the fiberboard sheet 5A, as shown in FIG. 10. Therefore, this cause warp of the fiberboard sheet 5A along the width direction, also lowering the quality of the resultant double-faced corrugated fiberboard 5.
To solve the foregoing problems, Patent Literature 1 discloses, in the specification and the drawings, a configuration of a heat plate in which providing many parallel holes through which heating medium is supplied inside the bulkhead of the heat plate to thin the bulkhead between the holes and the surface on which sheets travel. This configuration enhances and also equalizes the heat dissipating efficiency to the surface on which sheets travel and facilitates the adjustment of heating. FIG. 5 of Patent Literature 1 discloses a heat-plate structure having a number of reinforcing ribs on the bottom side of the heat plate.
Patent Literature 2 discloses a technique in which a heat plate is thinned and many stays that prevent the heat plate form thermal deformation are provided to the bottom side of the heat plate so that the rigidity of the stays prevents thin heat plate from warping.
Patent Literature 3 discloses a technique in which providing many parallel holes through which heating medium is supplied inside the bulkhead of the heat plate to thin the bulkhead between the holes and the surface on which sheets travel and concurrently, many ribs are provided to the bottom side of the heat plate, and holes through which heating medium is supplied are also provided to the ribs. The heating medium in the holes of the heat plate is supplied to the holes of the ribs, so that the temperature of both the heat plate and the ribs is concurrently adjusted.    [Patent Literature 1] The specification and a drawing (FIG. 5) of Japanese Utility-Model Laid-Open Publication No, HEI 2-48329    [Patent Literature 2] U.S. Pat. No. 5,417,394    [Patent Literature 3] U.S. Pat. No. 5,183,525